docs: def watch_docs(ctx):
      """Run build the docs when a file changes."""
      try:
          import sphinx_autobuild  # noqa
      except ImportError:
          print('ERROR: watch task requires the sphinx_autobuild package.')
          print('Install it with:')
          print('    pip install sphinx-autobuild')
          sys.exit(1)
      docs(ctx)
      ctx.run('sphinx-autobuild {} {}'.format(docs_dir, build_dir), pty=True)